1. Field of the Invention
The present invention relates to rotary piston engines, and more particularly, to rotors for rotary piston engines. More specifically, the present invention pertains to rotors having plastic coatings on their flank surfaces.
2. Description of the Prior Art
In the field of rotary piston engines, it has already been proposed to provide plastic coating on flanks of rotors. For example, Japanese patent publication 49-47207 published for opposition on Dec. 14, 1974 teaches to form a soft plastic coating at least on the leading part of each rotor flank. The purpose of such plastic coating is to decrease the gap between the inner wall of the rotor housing and the rotor flank to thereby decrease the amount of combustion gas carried over from the exhaust working chamber to the intake chamber. The Japanese patent publication suggests to use polyamide resin, phenolic resin, tetrafluoroethylene or silicon rubber, which is applied to the rotor flank and then dried under an elevated temperature. The proposed structure is considered as being effective to suppress engine knocking due to self ignition of the combustion mixture since the amount of the residual combustion gas can be decreased.
It should, however, be noted that the proposed structure has a danger of carbon lock, which is a phenomen wherein the rotor is clogged by carbon-containing particles which are caught in the gap between the rotor and the rotor housing. Usually, in operation of rotary piston engines, fuel and a small quantity of lubricant oil are burnt in the working chambers and produce unburnt residual substances. Such residual substances include organic ones such as tar and other carbon-containing substances, and inorganic ones such as sulphuric acid, sulphur, lead and other substances. These residual substances deposit on the rotor flanks and are forced under the pressure of the combustion gas in the working chamber to the rotor flanks to form deposit layers, which are similar to the coatings formed through spray coating processes. In the deposits of such residual substances the inorganic components have a tendency to absorb moisture in air and to swell to weaken the adhesive properties of the deposited layer. If such moisture absorption is repeated, the deposited layer is broken and broken pieces are allowed to fall off.
In conventional rotary piston engines, the rotor surfaces are usually galvanized and the deposits of the residual substances are adhered to the galvanized layer through hydrogen bonds. It should further be pointed out that an acidic atmosphere is therefore formed at the interface between the deposited layer and the galvanized layer and the acidic atmosphere is effective to maintain the hydrogen bond. However, moisture that penetrates to the interface promotes oxidation of zinc producing hydroxides, which are of an alkaline nature. Such hydroxides function to neutralize the acidic atmosphere and form zinc soap layers which have very low adhesive power. Thus, the aforementioned carbon lock is very likely to occur in rotary piston engines having galvanized rotors.
In the aforementioned Japanese patent publication, the rotor flanks are coated by resin which is simply applied thereto and dried under an elevated temperature. The resin coating has little adhesive property so that deposits of the unburnt residual substances are very easily removed from the rotor flanks possibly causing the carbon lock. In fact, depending on the lubricating oil and/or the fuel, such carbon lock phenomenon occurs at a very early stage of the engine life.